The present invention relates generally to printing in image forming systems, and specifically relates to duplex printing in such systems.
Conventional image forming systems, such as toner imaging systems, where a latent charge image is developed with a pigmented toner, are widespread in the office and home. Once developed with the toner, the image is transferred to a receiving member to form a printed image on a substrate, such as a sheet of paper.
Many technologies exist for forming a latent charge image, including optical image projection onto a charged photoconductive belt or drum, charging a dielectric member with an electrostatic pin array or electron beam, and charge projection from an ionographic print cartridge or from a plasma generator. Once a latent image is formed, the latent image may be transferred to an intermediate member before development. Alternatively, the latent image may be developed on the same member as that on which it is formed, with different system architectures having evolved to address different process priorities, such as cost, speed, preferred type of toning system or intended receiving substrate. A liquid-carried toner or a dry powder toner may be used. The former raises environmental issues that involve solvent or carrier management, especially when printing on so-called plain, or bond, papers, while the latter raises concerns of dust control, especially as the toner particle size becomes finer.
In general, there are two methods of producing the final image on a substrate. First, according to a conventional heating method, the toned image, once transferred to a receiving member, is heated to dry or fix the image on the substrate during the final stage of printing. Heating of the toned image at an earlier stage, e.g., when the toner is applied as a dust or liquid suspension to the latent charge image, is, however, avoided. In addition, in the heating method, heating should also be avoided on or near any photoconductive elements. Even for charge deposition systems in which an electric charge is applied to a dielectric rather than photoconductive member, heat may impair the dielectric properties of some common image-holding materials.
Aside from the sensitivity of the components of the system to heat, one disadvantage associated with the heating method arises when trying to print in duplex mode where an image is formed on both sides of the substrate, which, for example, may be paper. In the cut-sheet environment, where the substrate is a cut sheet of paper, the paper is re-circulated in a printing machine to print on both sides. Unfortunately, recirculation increases the amount of time to print, and makes it more likely that paper jams can occur. In the web environment, where the substrate is an uncut roll of paper, printing in duplex mode is done by two printing stations, which can be separated by several meters. Such a method of printing on both sides of the substrate that involve two printing stations separated by such a distance can give rise to paper wrinkling, web breaks, problems registering the front page to the back page, and large xe2x80x9cfootprints.xe2x80x9d
The second method of producing the final image on a substrate is a transfusing method in which the toned image is simultaneously transferred to and fixed on the final member in a softened state. By controlling the temperature, the relative tackiness or the cohesion of the heated toner may be made to vary to achieve optimal transfer of the image between surfaces, and when transferring to a final recording sheet, to optimize xe2x80x9cimage fixxe2x80x9d properties.
The transfusing method of the present invention possess several advantages over the conventional image transfer and fusing methods. For example, because in the former the image is transferred and fused to the substrate simultaneously, there is a savings in both space and equipment to form a completed image on the substrate. It is a significant aspect of the present invention that the image forming system is capable of duplex printing using the transfusing method.
An image forming system for duplex printing is provided which transfers and fuses the duplex images to both sides of a substrate at a single transfuse nip. Part of the image forming system has reflection or mirror-image symmetry about a line formed by the substrate. On each side of the line of symmetry there is a simplex transfuse engine arranged mechanically to transfuse an image to both sides of the substrate at a single nip.
In particular, an image forming system for printing on both sides of a substrate is provided. The system includes first and second transfer members forming a single transfuse nip therebetween. The system also includes a first imaging member for generating a first toner image that is received by the first transfer member; and a second imaging member for generating a second toner image that is received by the second transfer member. At the single transfuse nip, the first transfer member is suitable for transferring the first toner image to a first side of the substrate to form a first print, and the second transfer member is suitable for transferring the second toner image to a second side of the substrate to form a second print.
The first transfer member exerts a first force on the substrate to form the first print, and the second transfer member exerts a second force on the substrate to form the second print, such that the first force and the second force simultaneously oppose each other. The first print and the second print may be formed simultaneously on the substrate at the single transfuse nip. The first and second transfer members may each have a surface energy of between about 20 and about 40 dynes/cm, and a hardness of between about 50 and about 80 Shore A. Moreover, the toner may have a softening temperature Ts. The first imaging member and the first transfer member may each operate substantially isothermally at temperatures T1 and T2, respectively. Likewise, the second imaging member and the second transfer member may each operate substantially isothermally at temperatures T1 and T2, respectively, such that T1 less than Ts less than T2.
The first transfer member may transfer the first toner image in a melted state to the first side of the substrate. Likewise, the second transfer member may transfer the second toner image in a melted state to the second side of the substrate, as the temperature of the first toner image and the second toner image decrease. The system may further include a preheat assembly for preheating the substrate to a temperature T3 prior to introduction to the transfuse nip, such that T3 less than T2.
An image forming system is also described herein that includes a first print engine for forming a first toner image, and a second print engine for forming a second toner image. The system further includes a single transfuse nip formed between the first and second print engines wherein the first and second toner images are transferred to opposite sides of a substrate at the nip.